Conventionally, there has been proposed a display device with a photosensor that, by incorporating a photodetection element such as a photodiode inside a pixel, can detect the brightness of external light and pick up an image of an object that is located close to the display. Such a display device with a photosensor is envisioned to be used as a bidirectional communication display device or display device with a touch panel function.
In a conventional display device with a photosensor, when using a semiconductor process to form known constituent elements such as signal lines, scan lines, TFTs (Thin Film Transistors), and pixel electrodes on an active matrix substrate, photodiodes or the like are formed at the same time on the active matrix substrate (see PTL 1 and NPL 1).
FIG. 59 shows an example of a conventional photosensor (PTL 2 and 3) formed on an active matrix substrate. The conventional photosensor shown in FIG. 59 incorporates a photodiode D1, a capacitor C2, and a thin-film transistor M2. The anode of the photodiode D1 is connected to wiring RST, which is for supplying a reset signal. The cathode of the photodiode D1 is connected to one of the electrodes of the capacitor C2 and the gate of the thin-film transistor M2. The drain of the thin-film transistor M2 is connected to wiring VDD, and the source thereof is connected to wiring OUT. The other electrode of the capacitor C2 is connected to wiring RWS, which is for supplying a readout signal.
In this configuration, the reset signal and the readout signal are respectively supplied to the wiring RST and the wiring RWS at predetermined times, thus enabling obtaining sensor output VPIX that is in accordance with the amount of light received by the photodiode D1. A description will now be given of operations of the conventional photosensor shown in FIG. 59, with reference to FIG. 60. Note that in FIG. 60, the reset signal at low level (e.g., −7 volts) is shown as VRST.L, the reset signal at high level (e.g., 0 volts) is shown as VRST.H, the readout signal at low level (e.g., 0 volts) is shown as VRWS.L, and the readout signal at high level (e.g., 15 volts) is shown as VRWS.H.
First, when the high level reset signal VRST.H is supplied to the wiring RST, the photodiode D1 becomes forward biased, and the potential VINT of the gate of the thin-film transistor M2 is expressed by Equation (1) below.VINT=VRST.H−VF  (1)
In Equation (1), VF is the forward voltage of the photodiode D1. Since VINT is lower than the threshold voltage of the thin-film transistor M2 at this time, the thin-film transistor M2 is in a non-conducting state in the reset period.
Next, the reset signal returns to the low level VRST.L (time t=RST in FIG. 60), and thus the photocurrent integration period (sensing period indicated by TINT shown in FIG. 60) begins. In the integration period, a photocurrent that is proportionate to the amount of light that has been incident on the photodiode D1 flows out of the capacitor C2 to discharge the capacitor C2. Accordingly, the potential VINT of the gate of the thin-film transistor M2 when the integration period ends is expressed by Equation (2) below.VINT=VRST.H−VF−ΔVRST·CPD/CT−IPHOTO·TINT/CT  (2)
In Equation (1), ΔVRST is the pulse height of the reset signal (VRST.H−VRST.L), IPHOTO is the photocurrent of the photodiode D1, and TINT is the length of the integration period. CPD is the capacitance of the photodiode D1. CT is the sum of the capacitance of the capacitor C2, the capacitance CPD of the photodiode D1, and the capacitance CTFT of the thin-film transistor M2. In the integration period as well, VINT is lower than the threshold voltage of the thin-film transistor M2, and therefore the thin-film transistor M2 is in the non-conducting state.
When the integration period ends, the readout signal rises at a time t=RWS shown in FIG. 60, and thus the readout period begins. Note that the readout period continues while the readout signal is at the high level. At this point, the injection of charge into the capacitor C2 occurs. As a result, the potential VINT of the gate of the thin-film transistor M2 is expressed by Equation (3) below.VINT=VRST.H−VF−ΔVRST·CPD/CT−IPHOTO·TINT/CT+ΔVRWS·CINT/CT  (3)
ΔVRWS is the pulse height of the readout signal (VRWS.H−VRWS.L). Accordingly, since the potential VINT of the gate of the thin-film transistor M2 becomes higher than the threshold voltage, the thin-film transistor M2 enters the conducting state and functions as a source follower amplifier along with a bias thin-film transistor M3 provided at the end of the wiring OUT in each column. In other words, the sensor output voltage VPIX from the thin-film transistor M2 is proportionate to the integral value of the photocurrent of the photodiode D1 in the integration period.
Note that in FIG. 60, the solid line waveform indicates changes in the potential VINT in the case where a small amount of light is incident on the photodiode D1, and the broken line waveform indicates changes in the potential VINT in the case where light at the saturation level has been incident on the photodiode D1. In FIG. 60, ΔVSIG is the potential difference proportionate to the amount of light that has been incident on the photodiode D1. ΔVINT in FIG. 60 is the upthrust amount of the potential VINT due to the readout signal being applied from the wiring RWS to the photosensor in the readout period.